Structural rearrangements involving the short arm of chromosome 12 occur in 10% of cases of childhood acute lymphoid leukemia. The translocation t(12;17)(p13;q21), an uncommon 12p abnormality, was identified in five of 2620 cases (0.2%) successfully karyotyped by the Pediatric Oncology Group or St Jude Children's Research Hospital. All five cases were classified as early pre-B; however, CD10 (common acute lymphoblastic leukemia antigen) was expressed at lower levels than other markers of B-cell lineage. Two cases also expressed the myeloid-associated antigen CD33. Leukemic cells were pseudodiploid in four cases, with an extra chromosome 21 in the fifth case. All of these patients achieved complete remission. Two relapsed during subsequent therapy, and three remain in continuous remission for ≥ 20 months.